When client logs in to a system, he gets routed to one instance ( or new one created for him ), my first question during using the system on that instance, will he gets switched/routed to another instance ?
If he gets routed, how to prevent that?
There is no possibility to have 'sticky sessions' with GAE.
If your implementation depends on one client 'sticking' to the same instance, then your application is inherently not scalable.